1. Field of the Invention
The present invention relates generally to flow field management. More specifically, the present invention relates to systems, apparatus, program product, and methods for providing boundary layer flow control by the creation of separated flow structures using plasma actuators employing dielectric barrier discharge principles.
2. Description of the Related Art
Adverse fluid flows generated over aerodynamic surfaces can buffet and fatigue any downstream structures so exposed. Additionally, such flows can affect efficiency by increasing drag or resistance over the surface. Such adverse fluid flows can be generated at the fore body of an aircraft or other upstream structure, and damage control surfaces, engines, after body or empennage, nacelles, turrets, or other structures integrated into the airframe. Additionally, these adverse fluid flows can be ingested within engine air intakes or other like air inlets leading to poor performance and/or stalling of the aircraft engines.
In the past, aircraft components were designed to minimize the strength of adverse pressure gradient flaw fields to reduce the extent of or eliminate the separation of boundary layer flow from aircraft surfaces to reduce the destructive structural impact of separated flow on aircraft components and performance. This approach, however, limits design options and increases vehicle size, weight and cost. Alternatively, the components in the path of the adverse fluid flows were structurally hardened or replaced more frequently to avoid failures resulting from these stresses. Placing components, such as engines or control surfaces, in non-optimal positions in order to reduce these stresses often results in reduced vehicle performance. Similarly, adding structural weight to support increased stress loads caused by the flow field vortices also results in reduced vehicle performance.
One of the most commonly used methods to control local boundary layer separation, albeit within ducted systems, is the placement of vortex generators upstream of the layer separation within a natural fluid flow. Vortex generators are small wing like sections mounted on the inside surface of the ducted fluid flow and inclined at an angle to the fluid flow to generate a shed vortex. The height chosen for the best interaction between the boundary layer and the vortex generator has previously been the boundary layer thickness. The principle of boundary layer control by vortex generation relies on induced mixing between the primary fluid flow and the secondary fluid flow. The mixing is promoted by vortices trailing longitudinally near the edge of the boundary layer. Fluid particles with high momentum in the stream direction are swept along helical paths toward the duct surface to mix with and, to some extent, replace low momentum boundary layer flow. This is a continuous process that provides a source to counter the natural growth of the boundary layer creating adverse pressure gradients and low energy secondary flow accumulation. The devices, however, themselves cause drag which reduces the effectiveness of the devices.
Other potential solutions include the employment of active or passive control flows through mass injection using positive and/or zero mass devices to mitigate the effects of the adverse flow fields. These control jets manipulate the boundary layer, for example, through induced mixing between the primary fluid flow and the secondary fluid flow. The mixing is promoted by vortices trailing longitudinally near the edge of the boundary layer. Fluid particles with high momentum in the stream direction are swept along helical paths toward the aircraft surfaces to mix with and, to some extent replace low momentum boundary layer flow. This is a continuous process that provides a source to counter the natural deceleration of the flow near a solid surface in a boundary layer that can lead to flow separation in regions with adverse pressure gradients and low energy secondary flow accumulation. Mass injection devices utilizing a positive mass flow include, for example, passive jet spoilers which can utilize engine bleed air, ram air from an inlet or scoop, or an air/fluid pump. Such devices, however, require pneumatic/fluid conduits and/or manifolds to bring the control jets to regions requiring flow-control authority. Additionally, utilization of such devices result in added structural weight to supply and support the control jets, which results in reduced vehicle performance.
Various types of positive mass flow devices include combustion-driven jet actuators, which oxidize a gaseous fuel-air mixture. Specifically, such combustion-driven jet actuators include a combustion chamber that is filled with a combustible mixture which is then ignited, resulting in high pressures inside the chamber and mass expulsion through a chamber orifice. Besides the necessary fuel and air conduits, such devices also require a fuel storage capability, mechanical valves, and a means for igniting the fuel, which result in added structural weight to supply and support the control jets, which results in reduced vehicle performance.
Zero mass flow-capable devices include mechanical synthetic jets, single or dual bimorph synthetic jets, and spark jets. Synthetic jets, for example, which may be large scale devices or small scale Micro-fabricated Electro-Mechanical Systems (MEMS) devices, can be employed along an airfoil surface to control flow separation on the airfoil. A typical synthetic jet actuator includes a housing forming an internal chamber and an orifice in a wall of the housing. The actuator further includes a mechanism in or about the housing for periodically changing the volume within the internal chamber so that a series of fluid vortices are generated and projected into an external environment flow beyond the orifice of the housing. Various volume changing mechanisms include, for example, a reciprocating piston configured to move so that fluid is moved in and out of the orifice during reciprocation of the piston, and/or a flexible diaphragm forming one or more walls of the housing. In a similar device, the flexible diaphragm can instead be actuated by a piezoelectric actuator, such as, for example, one or more bimorph piezoelectric plates or other appropriate means connected by a flexible hinge or hinges.
Mechanical and bimorph synthetic jet actuators employing a flexible diaphragm typically include a control system is to create time-harmonic motion of the diaphragm. As the walls of the diaphragm (or diaphragms) move into the center of the chamber, the chamber volume decreases, and fluid is ejected from the chamber through a chamber orifice. As the fluid passes through the orifice, the flow separates at the sharp edges of the orifice and creates vortex sheets which roll up into vortices. These vortices move away from the edges of the orifice under their own self-induced velocity. As the vortices travel away from the orifice, they synthesize a jet of fluid, a “synthetic jet,” through entrainment of the ambient fluid. As the walls of the diaphragm move outward with respect to the center of the chamber, increasing the chamber volume, ambient fluid is drawn in from large distances from the orifice and into the chamber. These devices add additional weight and/or complexity to the air foil design.
The other aforementioned zero mass-capable device, a spark jet, can also be employed, for example, along an airfoil surface in a similar fashion to that of the mechanical or bimorph synthetic jets to control flow separation on the airfoil. Akin to the mechanical or bimorph synthetic jets, a typical spark jet also includes a housing forming an internal chamber and a chamber orifice in a wall of the housing. In contrast to the mechanical or bimorph synthetic jet actuators, however, the spark jet includes electrodes to produce an electrical discharge to heat the fluid within the internal chamber, which causes the fluid to accelerate out of the chamber orifice. The walls of the spark jet are generally relatively rigid in order to withstand the chamber pressure resulting from the rapid heating of the fluid within the chamber, without significantly deforming. The inner chamber pressure is relieved by the exhaustion of the heated fluid through the chamber orifice. Fluid is returned to the inner chamber through a corresponding decrease in pressure caused by cooling of the chamber walls and the gases remaining within the internal chamber upon removal of the current to the electrodes. As with the mechanical and bimorph synthetic jet actuators, the spark jet actuators also add additional weight and/or complexity to the air foil design.
Other potential solutions for controlling boundary layer separation and reducing drag include the use of devices which form a suction at the surface of the airfoil. Such devices, however, result in increased costs, added weight, and increased complexity to the overall system.
Still other potential solutions include surface air heating, for example, through use of plasma surface discharges, and/or use of dielectric-barrier discharge (“DBD”) type plasma actuators which produce a phenomenon often referred to as either an electrical or ionic wind across the surface of the airfoil. Plasma actuators, in their simplest form, consist of two electrodes, placed opposite of each other on a dielectric material. Discharges are created between the surface of the dielectric and the corresponding electrode by applying high frequency and high voltage. The resultant discharge creates a net displacement of the air that is near the dielectric surface layer; which creates a flow structure with a flow strength that is directly dependent on the applied power at the electrodes. There is an equal discharge created on the opposing electrode, which can be suppressed by isolating it from the surrounding air, such as, for example, by covering it with another electrode or an insulating material. The net result is an asymmetrical DBD with one electrode buried and not contributing to the net momentum, and the other electrode exposed on the surface being the single contributor to the momentum.
Velocity profiles of these simple DBD's have been researched and characterized by pitot-probe pressure measurements. Research by the inventors shows that the flow structure is generally uniform, directed away from the edge of the exposed electrode, at a maximum near the electrode, and with a pathway that follows the surface. Further, research by the inventors also shows that that these simple DBD constructs can impart a net momentum to the surrounding air producing a net force that acts on the DBD. Research by the inventors also shows that there is a small upward component to the velocity, but these constructs alone do not form velocities that are primarily upward. In addition, these constructs do not form individual regions that are individually addressable.
Moreau, in a paper titled “Air Flow Control by Non-Thermal Plasma Actuators,” J. Phys D: Appl. Phys. 40 (2007) pp. 605-636, incorporated herein by reference in its entirety, describes a potential configuration of a DBD-based actuator consisting of two annular-shaped electrodes, with the first air-exposed and the second embedded in order to form a plasma jet actuator having a velocity perpendicular to the device. I.e., the device consists of two stacked ring or washer-shaped electrodes with the diameter of the bottom electrode being smaller than the diameter of the top electrode, but larger than the inner diameter of the top electrode. Notably, although it is believed by the inventors that such annular-shaped device would provide some functionality if arranged according to various delivery patterns according to various embodiments of the present invention and/or interfaced with control systems described hereinafter and if exposed to air flows with relatively low Reynolds numbers, it is expected that such would not be effective at higher Reynolds numbers. Recognized by the inventors is that at higher Reynolds numbers, efficient operation generally dictates that plasma actuators need to be larger and spaced at a much closer interval than at lower Reynolds numbers. As such, at a higher Reynolds number, the annular design would be less desirable, as the annular shape would be expected to dictate a requirement for spacing between actuators that would be larger than desired.
Accordingly, recognized by the inventors is the need for flow control systems, apparatus, devices, electrode assemblies, controllers, program product, and methods: which provide DBD-based plasma actuators that are capable of providing a vertical flow stream under flow conditions having relatively higher Reynolds number values and which are precisely shaped and positioned to provide for such flow conditions; which produce separate regions of flow structures at different strengths by means of dielectric-barrier-discharge (DBD) type plasmas; which provide plasma regions that are capable of being individually controlled by voltage and/or frequency, modulated and/or unmodulated, for the purposes of flow control; which provide electrode assemblies having electrodes on either side of a dielectric so that different electrode geometries and arrangements create isolated regions of plasmas which result in separate regions of flow structures that may be further controlled and modulated by the use of electronic-switching to produce irregularly shaped flow structures and strengths to adjust for different flow conditions; and which are precisely sized to withstand voltages necessary to achieve flow control.
Also recognized by the inventors is the need for methods of forming such apparatus, devices, electrode assemblies, and controllers, which include application of electrodes by techniques, such as sputtering, which minimize surface thickness and roughness, resulting in drag caused by the plasma actuators themselves, which allows for a specified electrode geometry and arrangements as well as precision to create opposing electrodes, which provides a desirable quality in flows across surfaces. Also recognized by the inventors is that additional sputtering of other materials could be used to suppress unwanted discharge regions, such as, for example, those caused by corners or other edges in the electrodes.